1. Field of the Invention
The present invention relates generally to warning light devices, and more particularly to shallow depth, large area light assemblies and to warning light assemblies incorporating an LED light source.
2. Description of the Related Art
The prior art contains numerous examples of alternative light sources, reflectors and lenses arranged to produce particular intensities and distributions of light suited for a particular purpose. Of primary concern to designers of lights are the related concepts of efficiency and illumination distribution. By efficiency, it is meant that lighting designers are concerned with both producing the maximum quantity of light (lumens) per unit of energy (watts of electricity) and transforming that light into a useful pattern with minimal losses. Distribution refers to the precision with which a light fixture arranges the light into a desired pattern. The concept of efficiency is related to the concept of distribution because light that is scattered, e.g., not accurately directed in the desired pattern, is effectively lost by being dispersed.
Until recently, light-emitting diodes (LEDs), while recognized as efficient producers of light in terms of lumens per watt, were extremely limited in the overall quantity of light produced, rendering them unsuitable for many applications. Further, typical LEDs had a very narrow viewing angle, making them appear as point light sources unsuitable for many applications. “Viewing angle” as used herein refers to the angle, measured with respect to an axis through the center of the lens of the LED, where the light intensity has fallen to fifty (50%) of the on-axis intensity. For example, a very bright LED, producing 3 to 5 candela on axis may have a very narrow viewing angle of 8 to 15 degrees.
Recent advances in LED technology have resulted in LEDs having significantly improved overall light output. High-output (high flux) LEDs may now be a practical light source for use in signaling and warning illumination. Even though high-output LEDs have significantly greater luminous flux than previous LEDs, the total luminous flux from any given component is still relatively small, e.g., in the range of 5 to 20 candela. Modern, high flux LEDs have a wide viewing angle of 110 to 160 degrees. Thus, these newer LEDs produce a “half globe” of light in contrast to a directed “spot” of light with the older LEDs. For many applications, it may be necessary to accumulate multiple LEDs in a compact array and organize their cumulative light output to produce a light unit having an output pattern of a required size and intensity.
LEDs are attractive to lighting designers for certain applications because the light they produce is typically of a very narrow spectral wavelength, e.g., of a single pure color, such as red, blue, green, amber, etc. LEDs are extremely efficient producers of colored light because the particular chemical compound used in the die of the LED, when excited by electrical current, produces a monochromatic band of energy within the visible light spectrum. For example, a red LED will generate a narrow wavelength of light in the visible red spectrum, e.g., 625 nm+/−20 nm. No external color filtering is needed, significantly improving the efficiency of the light source. Further, LEDs are directional light sources. The light produced from an LED is primarily directed along an optical axis through the center of the lens of the LED. However, and in particular with the more recent high-output LEDs, a significant portion of the light is also directed out the sides of the lens of the LED (the above mentioned “half globe”). Accordingly, if the limited light output of an LED is to result in a practical signaling or illuminating device, as much of the light produced by each LED must be captured and directed in the desired light pattern as possible.
U.S. Pat. No. 6,318,886, assigned to the assignee of the present invention, discloses a high flux LED light assembly using conical reflectors. The conical reflectors disclosed in the '886 patent redirect light incident upon them out the face of the light assembly over a range of angles because the direction of the reflected light depends on the angular relationship between incident light and the reflecting surface. Such an arrangement, while desirably redirecting light out the front face of the assembly, undesirably does so over a range of angles, albeit a narrower range of angles than an LED in the absence of the conical reflector. Some of the reflected light reinforces light output of the LED. Other light is reflected at random angles that fail to reinforce the light output of the LED and is effectively lost by being dispersed. The light pattern produced is essentially a series of bright points of light having somewhat improved wide-angle visibility due to grooves connecting adjacent conical reflectors.
It is known in the art to use parabolic reflectors to collimate the light output from prior art light sources such as halogen bulbs or xenon flash tubes. U.S. Pat. Nos. 4,792,717 and 4,886,329, both directed to a wide-angle warning light and both assigned to the assignee of the present invention, disclose the use of a parabolic reflector comprised of a linear parabolic section including parabolic dish ends. The reflector is configured with a reflecting surface having a linear focal axis similar in configuration to the extended length of the xenon flash tube light source.
U.S. patent application Ser. No. 10/081,905, assigned to the assignee of the present invention, discloses an LED light assembly in which a linear array of equidistantly spaced high flux LEDs are arranged along the linear focal axis of a reflector having a linear parabolic section. Light emitted from the several high flux LEDs is allowed to overlap and combine while the linear parabolic reflector redirects the light into a wide angle band of light. The disclosed arrangement uses a steep parabolic reflecting surface having a short focal length. The short focal length of the reflecting surface permits mounting the LED array to the rear of the reflector. The parabolic reflecting surface redirects the off axis light from the LEDs into a partially collimated wide-angle beam. The resulting light pattern resembles a band of light with good visibility over a horizontal arc of approximately 90°.
Although LED light sources exhibit significant advantageous characteristics, replacing warning and signal light sources in warning arrays produced before the advent of the high flux LED with LED light sources is far from straightforward. To be cost-effective, LED replacement light units must have the same structural envelope and similar power requirements as the previous halogen or xenon flash tube light units. In other words, the LED replacement unit must have a similar height, width and depth to fit in the space allotted for the halogen or xenon light unit so that replacement does not require modification of the warning array which typically has an efficiently integrated structure with sophisticated functional capabilities. Thus, providing LED light units that are direct replacements for pre-existing light units designed around other light sources presents significant technical challenges.
Accordingly, there is a need in the art for a light emitter unit incorporating an LED light source that is a direct replacement for light emitter units pre-dating the advent of the high flux LED.